Currently 2D crystals are being studied intensively for use in future nano-electronics, as conventional semiconductor devices face challenges in high power consumption and short channel effects when scaled to the quantum limit. Toward this end, achieving barrier-free contact to 2D semiconductors has emerged as a major roadblock. In conventional contacts to bulk metals, the 2D semiconductor Fermi levels become pinned inside the bandgap, deviating from the ideal Schottky-Mott rule and resulting in significant suppression of carrier transport in the device. Here we realized MoS 2 polarity control without extrinsic doping by employing 1D elemental metal contact scheme. Use of high work function palladium (Pd) or gold (Au) achieved high quality p-type dominant contact to intrinsic MoS 2 , realizing Fermi level de-pinning. Field-effect transistors (FET) with Pd edge contact and Au edge contact show high performance with the highest hole mobility reaching 330 cm 2 /Vs and 432 cm 2 /Vs at 300 K respectively. The ideal Fermi level alignment allows creation of p-and n-type FETs on the same intrinsic MoS 2 flake using Pd and low work function molybdenum (Mo) contacts, respectively. This device acts as an efficient inverter, a basic building block for semiconductor integrated circuits, with gain reaching 15 at V D =5 V.
We introduce a multi-scale approach to obtain accurate atomic and electronic structures for atomically relaxed twisted bilayer graphene. High-level exact exchange and random phase approximation (EXX+RPA) correlation data provides the foundation to parametrize systematically improved force fields for molecular dynamic simulations that allow to relax twisted layered graphene systems containing millions of atoms making possible a fine sweeping of twist angles. These relaxed atomic positions are used as input for tight-binding electronic band-structure calculations where the distance and angle dependent interlayer hopping terms are extracted from density functional theory calculations and subsequent representation with Wannier orbitals. We benchmark our results against published force fields and widely used tight-binding models and discuss their impact in the spectrum around the flat band energies. We find that our relaxation scheme yields a magic angle of twisted bilayer graphene consistent with experiments between 1.0 • ∼ 1.1 • using commonly accepted Fermi velocities of graphene υF 1.0 ∼ 1.1 × 10 6 m/s that is enhanced by about 14%∼20% compared with often used local density approximation estimates. Finally, we present high-resolution spectral function calculations for comparison with experimental ARPES. Additional force field parameters are provided for hBN-layered materials.
Two-dimensional heterostructures composed of layers with slightly different lattice vectors exhibit new periodic structure known as moiré lattices, which, in turn, can support novel correlated and topological phenomena. Moreover, moiré superstructures can emerge from multiple misaligned moiré lattices or inhomogeneous strain distributions, offering additional degrees of freedom in tailoring electronic structure. High-resolution imaging of the moiré lattices and superstructures is critical for understanding the emerging physics. Here, we report the imaging of moiré lattices and superstructures in graphene-based samples under ambient conditions using an ultrahigh-resolution implementation of scanning microwave impedance microscopy. Although the probe tip has a gross radius of ~100 nm, spatial resolution better than 5 nm is achieved, which allows direct visualization of the structural details in moiré lattices and the composite super-moiré. We also demonstrate artificial synthesis of novel superstructures, including the Kagome moiré arising from the interplay between different layers.
Despite recent efforts for the development of transition-metal-dichalcogenide-based high-performance thin-film transistors, device performance has not improved much, mainly because of the high contact resistance at the interface between the 2D semiconductor and the metal electrode. Edge contact has been proposed for the fabrication of a high-quality electrical contact; however, the complete electronic properties for the contact resistance have not been elucidated in detail. Using the scanning tunneling microscopy/spectroscopy and scanning transmission electron microscopy techniques, the edge contact, as well as the lateral boundary between the 2D semiconducting layer and the metalized interfacial layer, are investigated, and their electronic properties and the energy band profile across the boundary are shown. The results demonstrate a possible mechanism for the formation of an ohmic contact in homojunctions of the transition-metal dichalcogenides semiconductor-metal layers and suggest a new device scheme utilizing the low-resistance edge contact.
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